New synthesis of μ-allenylidene complexes from dimolybdenum carbenium ions crystal structure of [Mo(η5-C5H5)(CO)22(μ, η2-CCC6H10)]

Abstract A selective abstraction of the acetylenic proton of cationic allynyl complexes [MoCp(CO) 2 2 ( μ - η 2 , η 3 -HCCCR 1 R 2 )][BF 4 ](Cp η 5 -C 5 H 5 ) by using the acetylide LiCCC(CH 3 )CH 2 gives rise to allenylidene complexes [Cp 2 Mo 2 (CO) 4 ( μ , η 2 -CCCR 1 R 2 )] R 2 = C 6 H 5 ( 6c ). Compounds 3b and 4b exist as two isomers while for 6c the formation reaction is stereoselective. The crystal structure of [Cp 2 Mo 2 (CO) 4 ( μ , η 2 -CCC 6 H 10 )] ( 1b ) has been determined.